1. Technical Field
The present invention relates to a phase change random access memory (PRAM), and more particularly, to a phase change random access memory and a method for manufacturing the same.
2. Related Art
In general, a phase change random access memory (PRAM) stores data using a difference in resistance between an amorphous state and a crystalline state, by applying an electrical pulse to a phase change layer formed of a chalcogenide compound. The amorphous state of the phase change layer is obtained by applying a high current to increase the temperature of a phase change substance over a melting point and subsequently performing instant-cooling. The crystalline state of the phase change layer is obtained by generating a nucleus from applying low current in the amorphous state and undergoing a growing process.
Here, in the phase change random access memory, while change from the amorphous state into the crystalline state is easily implemented, change from the crystalline state into the amorphous state consumes large operating current.
In order to reduce operating current and improve reliability in the phase change random access memory, a method for forming a phase change layer as a confined structure may be used.
In forming the confined structure, a phase change area in the form of a hole is defined through a damascene process instead of etching, and a phase change substance is filled in the phase change area to increase phase change efficiency.
In using the confined structure, if phase change substances have uniform composition ratios in the hole, operating current increases since a phase change area is wide. In general, the phase is change layer is filled through chemical vapor deposition (CVD) or atomic layer deposition (ALD). In this regard, since a process is performed at a relatively high temperature (around 300° C.), a phase change area is widened due to interatomic diffusion and chemical reaction, and operating current increases.
Also, as the degree of integration of a phase change random access memory increases, the distance between cells decreases and disturbance is caused due to heat so that the reliability of the phase change random access memory deteriorates.
A phase change random access memory in which a phase change substance is embedded at a low temperature to reduce operating current and improve the reliability of a phase change random access memory and a method for manufacturing the same are described herein.
In one embodiment of the present invention, a phase change random access memory includes: a semiconductor substrate having a bottom electrode formed over the semiconductor substrate; and a phase change layer formed over the bottom electrode, wherein the phase change layer includes a first phase change layer formed over the bottom electrode and including at least one of a first element, a second element, and a third element; and a second phase change layer formed over a surface of the first phase change layer and formed of the first element to prevent an area of the first phase is change layer from increasing through diffusion.
In another embodiment of the present invention, a phase change random access memory includes: a semiconductor substrate having a bottom electrode formed over the semiconductor substrate; and a phase change layer formed over the bottom electrode, wherein the phase change layer includes a first phase change layer formed over the bottom electrode and including any one of a binary compound of a first element and a second element, a binary compound of the first element and a third element, and a binary compound of the second element and the third element; and a second phase change layer formed over a surface of the first phase change layer and including a binary compound of the first element and the second element or a binary compound of the first element and the third element, wherein the ratio of the first element in the second phase change layer is such that an area of the first phase change layer is prevented from increasing through diffusion.
In another embodiment of the present invention, a phase change random access memory includes: a semiconductor substrate having a bottom electrode formed over the semiconductor substrate; and a phase change layer formed over the bottom electrode, wherein the phase change layer includes a first phase change layer formed over the bottom electrode and including a ternary compound of a first element, a second element and a third element; and a second phase change layer formed over a surface of the first phase change layer and formed of a ternary compound of the first element, the second element and the third element, wherein the ratio of the first element is such that an area of the first phase change layer is prevented from increasing through diffusion.
In another embodiment of the present invention, a method for manufacturing a phase change random access memory includes: forming a bottom electrode over a semiconductor substrate; forming a first phase change layer over the bottom electrode using at least one of a first element, a second element, and a third element; and forming a second phase change layer over a surface of the first phase change layer using the first element.
In another embodiment of the present invention, a method for manufacturing a phase change random access memory includes: forming a bottom electrode over a semiconductor substrate; forming a first phase change layer over the bottom electrode using any one of a binary compound of a first element and a second element, a binary compound of the first element and a third element, and a binary compound of the second element and the third element; and forming a second phase change layer over a surface of the first phase change layer using a binary compound of the first element and the second element or a binary compound of the first element and the third element.
In another embodiment of the present invention, a method for manufacturing a phase change random access memory includes: forming a bottom electrode over a semiconductor substrate; forming a first phase change layer over the bottom electrode using a ternary compound of a first element, a second element and a third element; and forming a second phase change layer over a surface of the first phase change layer using a ternary compound of the first element, the second element and the third element.